docs: def _start(self):
          """Start the long running operation.
          On completion, runs any callbacks.
  
          :param callable update_cmd: The API reuqest to check the status of
           the operation.
          """
          try:
              self._polling_method.run()
          except Exception as err:
              self._exception = err
  
          finally:
              self._done.set()
  
          callbacks, self._callbacks = self._callbacks, []
          while callbacks:
              for call in callbacks:
                  call(self._polling_method)
              callbacks, self._callbacks = self._callbacks, []